A couple of ways you can do that, but they all have the same basic process. Probably the easiest way is to use the page layout properties to designate a different first page header/footer. That will automatically put a section break between the first page and the rest of the document (which is required, no matter how you do it). Then, on the first page, just paste the image into your header.
Alternatively, with a section break in place and the first section not linked to section two (look at the header/footer properties for section 2 and make sure that it's not set to "same as previous"), you can use the watermark feature (in Word 2003, it's Tools > Background > Printed Watermark). Just be sure to uncheck "washout" so that it's as bold as you want it to be.
One problem you might find is that the image may not be exactly the right size for the document, so you might have to play with the sizes a little to get it to look about right. Alternatively, you could adjust the margins so be within the border you have on your image.
